• 10 of 12 patients treated in HER2 expressing solid tumor Phase 1b trial were breast cancer patients

• Dose selected for ongoing Phase 2b clinical trial in Osteosarcoma was shown to be well tolerated in breast cancer patients

• Preclinical breast cancer efficacy models showed positive data to prevent breast cancer metastases to the brain, reduce tumor growth by over 90% in combination with HER2-targeted antibodies, and prevent the formation of breast cancer compared with controls

ROCKVILLE, Md., July 24, 2024--(BUSINESS WIRE)--OS Therapies, Inc. (NYSE-A: OSTX), a clinical-stage oncology-focused immunotherapy company developing cancer vaccines and antibody drug conjugate (ADC) therapeutic candidates, today announced positive data from a Phase 1 clinical trial of OST-HER2 in patients with HER2-expressing solid tumors in breast cancer and other cancers. Additionally, the Company announced positive preclinical efficacy data for OST-HER2 in multiple models of breast cancer.

The FDA has granted Rare Pediatric Disease Designation (RPDD), Orphan Drug Designation (ODD), and Fast Track Designation (FTD) for OST-HER2 in Osteosarcoma. OST-HER2, a biologic therapeutic candidate, is a Lm (Listeria monocytogenes) vector-based off-the-shelf immunotherapeutic vaccine designed to prevent metastasis, delay recurrence, and increase overall survival in patients with osteosarcoma and other solid tumors. The proposed OST-HER2 mechanism of action is based on innate and adaptive immune stimulating responses activated by the Lm vector. This treatment generates T cells that can eliminate or slow potential micrometastases that can grow into recurrent osteosarcoma and other solid tumors, including breast cancer. T cell responses home in on HER2 expressed by the tumor and then kill the cell, releasing additional tumor targets. There are currently no approved adjuvant treatments for recurrent osteosarcoma in the United States.

A total of twelve (12) patients with a history of HER2 positive cancer were enrolled in the Phase 1b clinical study, comprised of ten (10) patients with breast cancer, one (1) patient with esophageal cancer and one (1) patient with GE junction cancer. OST-HER2 was determined to be safe and well tolerated in patients with HER2-expressing solid tumors, including at the dose currently being used in the Company’s ongoing Phase 2b clinical trial in recurrent, resected osteosarcoma (AOST-2121, NCT04974008). The Company previously announced positive AOST-2121 interim data and is now in active discussions with the US FDA regarding a Breakthrough Therapy (BTD) designation application submitted for OST-HER2 based on the interim results. The Company is focused on approval for OST-HER2 in osteosarcoma, and thereafter plans to expand development into breast cancer and other HER2 expressing cancers such as esophageal cancer and colorectal cancer.